Units whose biological effect is based upon laser emission have lately found wide application in ophthalmologa. They ensure bloodless, contactless, microscopically accurate and strictly measured effect on tissues and medium of the eye globe which does not require any anesthesia.
At present two main, though basically different approaches to the use of laser ophthalmological units for treatment of eye diseases have taken shape. The first approach is connected with laser coagulation of various tissues and mediums of the eye globe. Laser ophthalmological units designed for such purposes are referred to as laser ophthalmocoagulators and their lasing sources are continuous wave lasers (vast majority are argon or krypton lasers; cf., for example, U.S. Pat. No. 3,720,130 filed Mar. 13, 1973) or pulsed free-running lasers (cf., for example, U.S.S.R. Author's Certificate No. 937,318/25-25 filed Jan. 8, 1965).
The clinical effect in such apparatuses is achieved through the thermal action of said lasers leading to coagulation of the tissue.
The second approach to the use of laser ophthalmological units is fundamentally different from the first one and is connected with the use of the laser beam for making holes, tearing of various portions of eye tissues etc. This is achieved through the use of Q-switching of the laser emission (so called giant pulse operation). The action of such laser emission cannot be defined by the term "coagulation" or "cauterization," since it is based on non-thermal effects of the laser beam caused by the great power of the laser pulse. Laser ophthalmological units of the second type are at present employed mainly for treatment of glaucoma (cf., for example, U.S. Pat. No. 3,884,236 filed May 20, 1975 by M. M. Krasnov under the title "Method of Treatment of Glaucoma by Laser").
Any laser ophthalmological unit comprises the following basic components: the laser proper, whose emission is directed to the part of the patient's eye to be treated; a slit illumination system for lighting and selecting the surgery field in the process of accurate steering of the laser beam to the preselected part of the patient's eye; a microscope for watching the surgery field and the position of the marker indicating the focus point of the laser beam; a laser emission supply and focusing system; lighting system providing a marker to steer the laser beam to the preselected part of the patient's eye.
Thus, for example, there is known a laser ophthalmological unit for treatment of glaucoma described in the U.S. Pat. No. 3,828,788 authored by M. M. Krasnov and others. This unit, apart from the above enumerated components of the lighting system, comprises an additional He-Ne laser which produces a marker on the surgery field and a special optical element to match the beam of this laser with the operating beam. Both lasers are rigidly connected and are a single unit mounted upon its own foundation, whereas the slit source of illumination is arranged separately and its beam is directed at an angle to the optical axis of the laser beam. The optical system for supply of the laser emission to the surgery field is rigidly connected to the optical microscopic observation system by attaching the focusing element directly to the casing of the microscope.
One of the main drawbacks of the above described unit consists in that the focusing element is rigidly attached to the casing of the microscope and is, therefore, located in front of the microscope lens. Such attachment, on the one hand, makes it impossible to move the slit illuminator in the opposite position with respect to the vertical plane of symmetry and this is evidently inconvenient for the operator when he is working in different peripheral portions of the eye globe and, in particular, when operating on both of the patient's eyes. Besides, the same reason makes impossible slit illumination along the observation axis which is a necessity in some clinical cases. On the other hand, rigid connection of the focusing element and the microscope lens make it impossible for the operator to direct the laser beam at different angles to the optical observation axis, which sometimes prevents efficient use of the unit when laser action is to be accompanied by observation in the optical section.
Another drawback of such a unit consists in the presence of a second laser, since it significantly complicates the design of the unit and increases its cost. Besides, the division of the laser unit and the slit source, as well as the necessity to converge two laser beams result in inevitable lengthening of the optical path of the operating laser beam, widening of its aperture due to divergence, increased aberrations of the optical system and, consequently, optical losses and wider local diameter of the laser beam.